It is known to use an apparatus for dyeing yarn of the type wherein yarn is wound on cones mounted on a perforate carrier in a vessel fitted with inlet and outlet conduits for the dye liquor, whereby the latter may be circulated in two modes, namely, in a first mode from the inside of the cones, and therethrough to the outside, and in a second mode vice-versa, that is to say from the outside of the cones and therethrough to the inside. In this known operation, ancillary systems may be present including an expression tank, a dyestuff liquor preparation tank, a heat exchanger and a control system.
Whether the dyestuff liquor is flowing in the first mode or in the second mode, the dyeing vessel is full of liquor, and the ratio of liquor to yarn may be approximately 8 liters liquor: 1 kg. yarn. The liquor flow may be approximately 20-40 liters/minute/kg. yarn in each direction.
It is known that in this type of operation, the dyeing time depends on the number of reversals of circulation of the dyeing liquor through the cones of yarn, and not merely on the residence time of the yarn in the vessel. It would therefore be advantageous to increase the back-and-fourth flow rate, so as to shorten the dyeing time. The obvious way to achieve such an object would be to increase the capacity of the pump(s) and piping. This method however is very expensive, because apart from the increased cost of the new and increased-capacity installation, the increased flow has to overcome increased suction, which increases in a squared relationship. This obvious method is therefore uneconomical.